Hi Daryl. I am guessing you have tried different cars and controllers in the 'dead' lane with the same result? And tried the powerbase separate from the rest of the track. That would eliminate whether it might be a car, controller or track issue. I'd also swap round the two transformers and move the switches on the powerbase - I think you have the older Scalextric Sport analogue powerbase? See if any changes can make the 'dead' lane work through the powerbase.
If all those changes don't make any difference, then there is an issue in the powerbase. There could be a short-circuit in the powerbase - try squeezing both controllers and see if the 'good' lane stops working. If so, check there is no metal debris in the slot of the 'dead' lane. You could also take the bottom plate off the powerbase (disconnect from the transformers first) and see if you can see any loose wires or any damage.
I hope something there helps. Do let us know.